Conventional optical wavelength dispersive devices, such as those disclosed in U.S. Pat. No. 6,097,859 issued Aug. 1, 2000 to Solgaard et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; U.S. Pat. No. 6,707,959 issued Mar. 16, 2004 to Ducellier et al; U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch; and U.S. Pat. No. 7,014,326 issued Mar. 21, 2006 to Danagher et al, separate a multiplexed optical beam into constituent wavelengths, and then direct individual wavelengths or groups of wavelengths, which may or may not have been modified, back through the device to a desired output port. Typically the back end of the device includes individually controllable devices, such as a micro-mirror array, which are used to redirect selected wavelengths back to one of several output ports, or an array of liquid crystal cells, which are used to block or attenuate selected wavelengths.
FIG. 1 illustrates a top view of a typical platform 102A for a wavelength dispersive device in which a light redirecting element having optical power in the form of a spherical reflector 120 receives a beam of light from a front-end unit 122. The spherical reflector 120 reflects the beam of light to a diffraction grating 124, which disperses the beam of light into its constituent wavelength channels. The wavelength channels are again redirected by the spherical mirror 120 to a backend unit 126.
In the case of a wavelength blocker (WB) or a dynamic gain equalizer (DGE) the front end unit 122 can include a single input/output port with a circulator, which separates incoming from outgoing signals, or one input port with one output port. Typically the front end unit 122 will include a polarization diversity unit for ensuring the beam (or sub-beams) of light has a single state of polarization. The backend unit 126 for a WB or a DGE is an array of liquid crystal cells, which independently rotate the state of polarization of the wavelength channels to either partially attenuate or completely block selected channels from passing back through the polarization diversity unit in the front end 122.
In the case of a wavelength selective switch (WSS) the front end unit 122 includes (See FIG. 2) an array 132 of input/output fibers 132A to 132D, each of which may have a corresponding lens 134A to 134D, respectively, forming a lens array 134. An angle to offset (or switching) lens 136 converts the lateral offset of the input fibers 132A to 132D into an angular offset at a point 138, which is imaged by the spherical lens 120 onto the backend unit 126. The lens array 134 can be removed depending on the relative positions of the switching lens 136. The backend unit 126 in an WSS is typically a micro-electro-mechanical (MEMS) array of tilting mirrors which can be used to steer each of the demultiplexed beams to one of several positions corresponding to a desired output port. The angle introduced at the back end unit 126 is then transformed by the angle to offset lens 136 to a lateral offset corresponding to the desired input/output fiber 132A to 132D. Alternatively, a liquid crystal phased array (LC or LCoS, if incorporated on a silicon driver substrate) can be used to redirect the light.
In operation as an WSS, a multiplexed beam of light is launched into the front-end unit 122 and optionally passes through a polarization beam splitter 138 and a waveplate 140A or 140B (See FIG. 3) to provide two sub-beams of light having the same state of polarization. The two sub-beams of light are transmitted to the spherical reflector 120 and are reflected therefrom towards the diffraction grating 124. The diffraction grating 124 separates each of the two sub-beams into a plurality of channel sub-beams of light having different central wavelengths. The plurality of channel sub-beams are transmitted to the spherical reflector 120, which redirects them to the MEMS or LC phased array 126, where they are incident thereon as spatially separated spots corresponding to individual spectral channels.
Each channel sub-beam can be reflected backwards along the same path or a different path, which extends into or out of the page in FIG. 1 to the array of fibers 132, which would extend into the page. Alternatively, each channel sub-beam can be reflected backwards along the same path or a different path, which extends in the plane of the page of FIG. 1. The sub-beams of light are transmitted, from the MEMS or LC phased array 126, back to the spherical reflector 120 and are redirected to the diffraction grating 124, where they are recombined and transmitted back to the spherical reflector 120 to be transmitted to a predetermined input/output port shown in FIG. 2.
FIG. 4 illustrates a conventional in-plane or horizontally switching platform, in which an input beam with optical wavelength channels λ1 and λ2 is launched via input/output port 31 through switching lens 35 to concave mirror 40. The input beam is redirected and collimated onto a diffraction grating 50, which laterally disperses the optical wavelength channels, and directs them at the concave mirror 40. Each optical wavelength channel is directed at and focused onto a different independently controllable micro-mirror, e.g. 61 and 62, which make up a MEMs array 60. The first optical wavelength channel λ1 is reflected straight back and therefore exits the input/output port 31, while the second optical wavelength channel λ2 is reflected at a predetermined angle corresponding to the lateral position of a second input/output port 32.
A transmission path correction element, i.e. wedge, 100, with front and rear non-parallel faces 101 and 102, respectively, is installed between the concave mirror 40 and the MEMS array 60. The purpose of this correction element 100 is to modify the paths of the optical signals focused by the concave mirror 40, so as to effectively rotate the best fit planar surface approximation FP into coplanar coincidence with the optical signal-receiving surface MP 65 of the MEMS array 60. Non-limiting examples of a suitable (field-flattening) transmission path correction element that may be used for this purpose include a portion or segment of a cylindrical lens and an optical transmission wedge. With the curvilinear focal surface LP of the spherical mirror 40 being transformed into a focal plane FP, and with that plane FP being coincident with the MEMS array plane MP 65, variation in loss, as minimized by the best fit linear approximation of the focal plane, will be effectively eliminated.
Unfortunately, each time a customer wishes to purchase a WB, a DGE, an MWS or any form of monitor therefor, they must purchase a separate dispersion platform, i.e. spherical lens and diffraction grating along with associated opto-mechanics and packaging. An object of the present invention is to overcome the shortcomings of the prior art by providing a multi-unit wavelength dispersive device, in which a plurality of independent front and backend units can utilize the same dispersion platform and share the same opto-mechanics and packaging.